Our aim is to study the organization of neurotransmitter receptors on nerve and muscle cells in relationship to the development and function of synapses. Our recent work has focused upon the factors, extinsic and intrinsic to the developing skeletal muscle fiber, which regulate the distribution of nicotinic acetylchone receptors and contribute to the assembly of a postsynaptic transmembrane complex. Acetylcholine receptor aggregation is induced on cultured myotubes by neuronal factors, and this system is used to study the mechanisms of receptor aggregation, as well as the stabilization or elimination of aggregates which occur in developing neuromuscular junctions. We use fluorescence and electron microscopy, as well as immunocytochemistry, to follow changes in the distribution of acetylcholine receptors and associated cell surface components, and to study the underlying ultrastructural changes. In the past year, we have found that: 1) Brain extract-induced acetylcholine receptor aggregates may form in regions of the cell surface previously enriched in the proteins vinculin and alpha-actinin. 2) The deposition of collagen and laminin, and the formation of muscle cell basal lamina induced by embryonic brain extract and ciliary ganglion are dependent on ascorbic acid or an ascorbate- like factor present in these neural preparations.